hold onto your hat

• hold on to your hat

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hold onto your hat

1. (idiomatic, usually humorous) Prepare for a shock!

   Synonym: hang onto your hat

   • 2011, Mark Ryan, Geometry Essentials For Dummies, John Wiley & Sons, →ISBN, page 14:

     Hold onto your hat—intersecting planes are planes that cross, or intersect.

2. Used other than figuratively or idiomatically: see hold,‎ onto,‎ hat.

• When addressing a group, the plural is used: hold onto your hats.

• “hold on to your hat”, in OneLook Dictionary Search.